L-methionine-γ-lyase in Citrobacter intermedius cells: Stereochemical requirements with respect to the thiol structure

Abstract

The specificity of l-methionine-γ-lyase with respect to the stereochemical structure of the thiol substrate in the γ-substitution reaction has been demonstrated. Cells of Citrobacter intermedius containing l-methionine-γ-lyase catalyze the exchange reactions between l-methionine and 2-propylthiol or 2-butylthiol which leads to the formation of s-2-propylhomocysteine and s-2-butylhomocysteine, respectively. The yields of these products are comparable to the yield of s-butylhomocysteine in the reaction of normal butylthiol with l-methionine, thus 2-propylthiol and 2-butylthiol are effective substrates of l-methionine-γ-lyase. On the other hand in the reaction of 3-pentylthiol, only traces of the expected product, s-3-pentylhomocysteine, were formed and in the case of 2-methyl-2-butylthiol, the expected product of γ-substitution, s-2-methyl-2-butylhomocysteine, was not formed at all. In the reaction with racemic 2-butylthiol, only one diastereomer of s-2-butylhomocysteine was obtained. The unreacted 2-butylthiol isolated after the reaction catalyzed by partially purified preparation of l-methionine-γ-lyase was enriched with ( R)-enantiomer which indicated the preferential reaction of the ( S)-enantiomer.